1. Field of the Invention
This invention relates to apparatus for generating computer data from the position of an object, such as might be used for determining the position of a stylus on a digitiser tablet.
2. Summary of the Prior Art
The stylus used on these tablets can be virtually any sort of pointing device. This could be in the form of a pen-like object, or a sliding puck with a cross-hair to indicate the sensitive point.
There are various known ways of obtaining x-y positional information in the form of computer data from a writing tablet. The stylus can be passive, writing on an active receiving surface (e.g. resistive and capacitative methods), or the stylus can be active, writing on a passive surface (e.g. optical, sound, infra-red, or electromagnetic), or a mechanical linkage can be used (e.g. the pantograph).
Passive stylus/active surface methods are often open to error by virtue of not being able to reliably distinguish between the stylus and any other pointed object. This problem can be overcome by having a manually operated switch to signal that a reading is to be taken. Usually, this switch is connected to the stylus nib, and so there must, by necessity, be some contact between the stylus and the drawing surface.
Active stylus/passive surface methods avoid the need for direct contact between the stylus and the drawing surface, but can be ineffectual if the transmitted signal from the stylus is obscured in some way before it is received by the detection device (which may not even be on the surface of the tablet). Of all these methods, electromagnetic induction has the desirable advantage of being relatively unaffected in its propogation by most intervening media, as well as avoiding the drawbacks of the passive techniques.
All these afore-mentioned methods are used only to measure the displacement across the "x" and "y" axes of the digitiser surface.
A known digitiser technology uses position sensing apparatus made up of two coil arrays which are electromagnetically coupled, one coil array being held within a drawing surface and the other coil being encased within a drawing stylus. If the drawing surface contains two such sets of rectangular coils, with the long axes of one set being perpendicular to the long axes of the other, then the apparatus can be used to generate the (x,y) coordinates of the stylus nib when it is close to the drawing surface.
A specially constructed coil array, housed within the stylus, may be used with such a sensing apparatus to provide data describing the rotation of the stylus and its degree of tilt from the perpendicular along the x-y axes.
An energised horizontal coil is positioned such that its axis lies parallel to those of an array of coils connected in series and alternately wound in opposite directions. A voltage will be induced in the latter coils whereby the phase and amplitude of the combined signal from those coils reflect whether the influencing magnetic field is over a clockwise- or anticlockwise-wound receiving coil. This arrangement is shown in FIG. 1 of the attached representations.
With the transmitting coil positioned at "A" in FIG. 1, the induced e.m.f of the combined signal will be phasecoherent with that of the transmitting coil. If the same transmitting coil is positioned over coil "B" the resulting e.m.f inverts the phase.
This method of position sensing requires additional information in order to reflect the position of the transmitting coil either side of any single coil in the receiving set. i.e. position "X" in FIG. 1 would cause the same resultant e.m.f as position "Y".
This ambiguity can be avoided by adding a second set of coils, identical to the first and interleaving them between the first set. See FIG. 2 of the attached representations.
It can be demonstrated that the resultant e.m.f generated for coil set in FIG. 1 is very closely proportional to: EQU SIN(wt)SIN(2.pi.d/P) (1)
then for the displaced coils we will have the same result except "d" will be displaced by P/4. The equation for e.m.f (2) generated will then be approximtely: EQU SIN(wt)SIN(2.pi.(d+P/4)/P)=SIN(2.pi.d/P+.pi./2).
Using identity EQU COS(A)=SIN(A+.pi./2)
so EQU e.m.f(2)=SIN(wt)COS(2.pi.d/P) (2).
Since the signal driving the transmit coil can be of a known phase then the resultant outputs from both sets of receiving coils can be regarded as being proportional to "d" only.
Thus there exists a pair of signals which represent the SIN and COS of the displacement of the source coil across pitch "P" of the receive coils. It is therefore possible to resolve "d" from these signals after they have been suitably demodulated, although demodulation is not strictly necessary.
The elongated nature of the sense coils allows the apparatus to operate as a practical digitising tablet and stylus by detection of field changes primarily across their short axes.
An apparatus of this nature is discussed in GB-A-1536123.
The drawback of this type of construction is that it is capable of generating only positional co-ordinate information and an indication of the degree of tilt of the stylus from the vertical. It is not possible to obtain information regarding the rotational position of the stylus.